The present invention relates to a zoom lens system composed of five lens groups.
A projector includes a light valve (image generating device) with LCD panels or the like, a projection zoom lens system, a dichroic prism or a TIR prism. The dichroic prism or the TIR prism combines rays expressing an image divided into colors outputted by the light valve. The projection zoom lens system needs to be telecentric on the image side for projecting the combined light onto a screen. For this reason, one of the types of the projection zoom lens system is a zoom lens system composed of five lens groups with negative, positive, positive, negative, and positive refractive powers disposed in that order from the screen side. During zooming, the first lens group and the fifth lens group would be fixed and the second, third, and fourth lens groups would move along the optical axis to change the distances between the lenses and in turn the size of the projected images.
A projection zoom lens system for a projector using LCD panel or panels needs to be telecentric on the image (input) side, to favorably correct various types of aberration, and also to be compact and have a predetermined back focus. To satisfy these demands, a suitable power balance for the respective lens groups should be found and the constructions of the respective lens groups should be determined.
In recent years, to make lens systems more compact, there has been a strong tendency to increase the power of the first lens group positioned closest to the screen, which makes it difficult to achieve a suitable focus adjusting performance. In more detail, in a mechanism where focus adjustments are made moving the first lens group, if the first lens group is too powerful, the aberration correcting performance will fluctuate according to changes of position of the first lens group, and therefore it becomes difficult to get favorable aberration correcting performance and favorable focus adjusting performance at the same time. Also, when the first lens group is powerful, the sensitivity of focus adjustments relative to distance will be too high, which makes focus adjusting operations by manual or mechanisms difficult. A mechanism that can overcome such difficulties would be so complex, with the mechanism, it would be difficult to realize a compact zoom lens system.
One of the important types of aberration that needs to be corrected in a zoom lens system is chromatic aberration. Cemented lenses that are respectively composed of a negative lens and a positive lens and have a high correcting performance for chromatic aberration are disposed in a number of lens groups so that the respective lens groups correct chromatic aberration together with the other types of aberration. However, compared to correcting an aberration of green light that has a substantially central frequency band of visible light, it is difficult to simultaneously correct aberrations of both red light that is low frequency light and blue light that is high frequency light suitably. Many lens systems attempt to correct red light during zooming using the second lens group that is moved during a zooming operation. To do so, the second lens group needs to have a fairly high refractive power, and to correct the aberration due to such refractive power, the first lens group also requires a fairly high refractive power. For this reason also, it is difficult to reduce the refractive power of the first lens group.
In the lens system described in Japanese Laid-Open Patent Publication No. 2002-148516, a cemented lens is provided in the third lens group as well as in the fourth lens group to reduce the chromatic aberration-correcting load of the second lens group. Such arrangement simplifies the construction of the second lens group and reduces the power of the second lens group and reduces the power of the first lens group in turn.